Tropical Medicine: A Clinical Text, 8th Edition, Revised and Expanded

Filariasis

Filariasis Filarial worms are long thread-like nematodes that parasitize the tissues of humans and animals. In the tropics, they include some of the most important parasites of humans. Except for Dracunculus medinensis, the life cycles are very similar. The adult worms live in the body for 15 years or longer during which time, the female produces enormous numbers of immature, first-stage larvae, 200 to 300 micrometers in length, known as microfilariae. The microfilariae do not develop further in the human body, but when taken up from the blood or skin by various arthropod vectors, develop into second- and third-stage larvae in the thoracic muscles of the insect host. The third-stage larvae enter the human body, completing the parasite life cycle, when the female arthropods take a blood feed. In the body, each infective larva may develop into a single male or female worm. Wuchereria bancrofti Infection Wuchereria bancrofti is the major cause of lymphatic filariasis, which is transmitted from man to man by Culicine, Anopheline, and Aedes mosquitoes in both urban and rural regions of the hot, moist tropics. In 1877, Patrick Manson, working in Amoy, China, noticed that microfilariae began to appear in the blood of infected persons at about sunset, gradually increasing toward midnight, and then gradually clearing . He persuaded his gardener, who was infected, to sleep in a ‘‘mosquito house’’; the next morning, he collected the blood gorged insects: I shall not easily forget the first mosquito I dissected. I tore off its abdomen and succeeded in expressing the blood contained in the stomach. Placing this under the microscope, I was gratified that, so PAGE 184 ................. 18086$ CH16 07-15-11 13:50:22 PS filariasis 185 far from killing the filariae, the digestive juices of the stomach of the mosquito seemed to have stimulated them to fresh activity. And now, I saw a curious thing; the little sack or bag enclosing the filariae, which hitherto muzzled it and prevented it from penetrating the wall of the blood vessels in the human body, was broken and discarded. In subsequent studies, Manson succeeded in tracing the filariae through the stomach wall, into the abdominal cavity of the mosquito, and then into the thoracic muscles. During this passage, the parasites increased enormously in size, developing a mouth, an alimentary tract, and other organs. This demonstration of transmission of parasites by arthropod vectors was to prove of fundamental importance in tropical medicine. The Parasite The female adult worm is a white thread-like nematode measuring 10 cm in length; the male is smaller at 4 cm. They lie coiled up in the lymphatic channels in a state of permanent copula, the female shedding microfilariae that reach the blood stream before they are extracted by mosquitoes. The rate of development in the thoracic muscles of the mosquito depends on temperature and humidity; at 79F and a humidity ⬍70%, it takes 12 days. The robust, sausage-shaped second stage larvae develop further into infective filariform organisms that escape from the mouth parts when the mosquito takes a blood meal. They are deposited on the human skin, which they penetrate when the proboscis is withdrawn. They rapidly enter the subcutaneous lymphatics and are carried to the regional nodes. After a few days, the larvae migrate back down the lymphatic trunks; over the course of the next 3 to 6 months, they mature through two molts to become male and female worms. It should be noted that, as with all helminths, there is no multiplication of adults in the human body. Therefore, the severity of the illness can be roughly correlated with the number of worms initially injected. Pathology In most parasitic diseases, tissue damage is part of an immune response of the body to the infecting organism. Filariae, however, appear to be PAGE 185 ................. 18086$ CH16 07-15-11 13:50:22 PS 186 filariasis able to suppress host reactions, thus allowing for prolonged life in humans. In indigenous populations of endemic regions, where exposure begins in infancy and continues until death, the only evidence of infection in the majority of affected individuals is a silent microfilaremia . There is often no other indication of disease, and levels of serum antibody are minimal or absent. In a minority of those infected, repeated attacks of lymphadenitis and lymphangitis occur, probably due to a host reaction to larval molting or from dead or...